Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-scaled graphene plates (NGPs). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. Jang (one of the instant applicants) and Huang have provided an indirect synthesis approach for preparing NGPs and related materials [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. In most of the prior art methods for making separated graphene platelets, the process begins with intercalating natural graphite particles with an intercalation agent (also known as an intercalant or intercalate), followed by thermally expanding the intercalant to exfoliate the particles. The exfoliation step produces graphite worms, which are networks of interconnected graphite flakes or platelets. In some methods, the graphite worms are then subjected to air milling, ball milling, or ultrasonication to separate the graphite flakes or platelets. Conventional intercalation and exfoliation methods and recent attempts to produce exfoliated products or separated platelets are given in the following representative references:    1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S. Pat. No. 3,434,917 (Mar. 25, 1969).    2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,” U.S. Pat. No. 3,885,007 (May 20, 1975).    3. A. Hirschvogel, et al., “Method for the Production of Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).    4. T. Kondo, et al., “Process for Producing Flexible Graphite Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).    5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No. 4,895,713 (Jan. 23, 1990).    6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat. No. 5,503,717 (Apr. 2, 1996).    7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997).    8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11, 2001).    9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).    10. M. Hirata and S. Horiuchi, “Thin-Film-Like Particles Having Skeleton Constructed by Carbons and Isolated Films,” U.S. Pat. No. 6,596,396 (Jul. 22, 2003).
However, these previously invented methods have several serious drawbacks. Typically, exfoliation of the intercalated graphite occurred at a temperature in the range of 800° C. to 1,050° C. At such a high temperature, graphite could undergo severe oxidation, resulting in the formation of graphite oxide, which has much lower electrical and thermal conductivities compared with un-oxidized graphite. Most of the prior art processes make use of undesirable acids (e.g., strong sulfuric acid+potassium chlorate) as an intercalant. Further, most of the prior art processes do not have a good control over the NGP dimensions. The approach proposed by Mack, et al. [e.g., Ref. 9] is a low temperature process. However, Mack's process involves intercalating graphite with potassium melt, which must be carefully conducted in a vacuum or an extremely dry glove box environment since pure alkali metals, such as potassium and sodium, are extremely sensitive to moisture and pose an explosion danger. This process is not amenable to mass production of nano-scaled platelets.
To address these issues, we have recently developed several processes for producing nano-scaled platelets, as summarized in several pending patent applications [Refs. 11-16]:    11. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. Pat. Pending, Ser. No. 11/509,424 (Aug. 25, 2006).    12. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Mass Production of Nano-scaled Platelets and Products,” U.S. Pat. Pending, Ser. No. 11/526,489 (Sep. 26, 2006).    13. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Method of Producing Nano-scaled Graphene and Inorganic Platelets and Their Nanocomposites,” U.S. Pat. Pending, Ser. No. 11/709,274 (Feb. 22, 2007).    14. Aruna Zhamu, JinJun Shi, Jiusheng Guo, and Bor Z. Jang, “Low-Temperature Method of Producing Nano-scaled Graphene Platelets and Their Nanocomposites,” U.S. Pat. Pending, Ser. No. 11/787,442 (Apr. 17, 2007).    15. Aruna Zhamu, Jinjun Shi, Jiusheng Guo and Bor Z. Jang, “Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled Graphene Plates,” U.S. Pat. Pending, Ser. No. 11/800,728 (May 8, 2007).    16. Aruna Zhamu, Joan Jang, Jinjun Shi, and Bor Z. Jang, “Method of Producing Ultra-thin, Nano-Scaled Graphene Plates,” U.S. Pat. Pending, Ser. No. 11/879,680 (Jul. 19, 2007).
For instance, Ref. [11,12] are related to processes that entail a pressurized gas-induced intercalation procedure to obtain a tentatively intercalated layered compound and a heating and/or gas releasing procedure to generate a supersaturation condition for inducing exfoliation of the layered compound. Tentative intercalation implies that the intercalating gas molecules are forced by a high gas pressure to penetrate into and reside tentatively in the interlayer spaces. Once the intercalated material is exposed to a higher temperature and/or a lower pressure environment, these gas molecules induce a high gas pressure that serves to push apart neighboring layers. Reference [13] is related to a halogen intercalation procedure, followed by a relatively low-temperature exfoliation procedure. No strong acid like sulfuric acid or nitric acid is used in this process (hence, no SO2 or NO2 emission) and halogen can be recycled and re-used. Disclosed in Ref. [16] is a method of producing ultra-thin NGPs that entail repeated intercalations and exfoliations of laminar graphite materials.
In all of aforementioned prior art methods and our co-pending applications, the process begins with intercalation of graphite-type materials, followed by gas pressure-induced exfoliation of the resulting intercalated graphite compound. The gas pressure is generated by heating and/or chemical reaction. However, intercalation by a chemical (e.g., sulfuric acid) may not be desirable. Exfoliation by heat can put graphite at risk of oxidation. After exfoliation, an additional mechanical shear treatment is normally needed to separate the exfoliated graphite into isolated platelets. In essence, every one of these processes involves three separate steps, which can be tedious and energy-intensive.
Furthermore, although prior art intercalation-exfoliation methods might be able to sporadically produce a small amount of ultra-thin graphene platelets (e.g., 1-5 layers), most of the platelets produced are much thicker than 2 nm (mostly thicker than 10 nm). The method provided in [Ref. 16] is the only exception, which is capable of consistently producing ultra-thin NGPs through repeated intercalations and exfoliations. Many of the NGP applications require the NGPs to be as thin as possible; e.g., as a supercapacitor electrode material. Hence, it is desirable to have a method that is capable of consistently producing ultra-thin NGPs.
Hence, it is an object of the present invention to provide a method of producing ultra-thin graphene platelets, with an average thickness smaller than 2 nm (or comprising less than 6 graphene layers per platelet).
It is a particular object of the present invention to provide a simple, fast, and less energy-intensive method of producing ultra-thin graphene platelets, without involving a long intercalation time, without using a high exfoliation temperature, or without a need for a subsequent mechanical shearing treatment (e.g., air milling).
It is another object of the present invention to provide a convenient method of producing nano-scaled graphene platelets without utilizing an undesirable intercalant, such as sulfuric acid.
Another object of the present invention is to provide an effective and consistent method of mass-producing ultra-thin, nano-scaled platelets.
It is still another object of the present invention to provide a method of producing ultra-thin, nano-scaled platelets that can be readily dispersed in a liquid to form a nanocomposite structure.